1. Field of the Invention
This invention relates generally to a protective circuit and more particularly to a protective circuit which effectively protects a transistor amplifier from an overload.
2. Description of the Prior Art
In the art there has been proposed a protective circuit for a transistor amplifier in which a load voltage and a load current are compared to detect a load impedance and in which when the load impedance becomes lower than a predetermined value, the transistor amplifier is stopped in function.
If the load is a reactive one, a phase difference occurs between the load voltage and load current. Accordingly, an error operation may appear in the comparing operation. A protective circuit, which can avoid such an error operation caused by the phase difference by comparing the load voltage with the load current after they are rectified, has been also proposed by the same applicant of this application, in copending application, Ser. No. 382,139.
In general, it is well known in the art that an undesirable DC voltage produced at the output terminal of a load-direct-coupled type amplifier damages a load such as a speaker.
Up to now, there has been proposed no protective circuit which will protect an amplifier from a short-circuiting and undesirable DC voltage positively with a simple circuit construction.